Every winter, we keep a bowl of walnuts on the table and the leftover empty shells just demand to be filled with something interesting- some years tiny clay dioramas, one year a pile of little, wobbly, Fair Isle knitted pincusions.... and this year tiny tortoises / turtles.
Most tiny knitted beasties are very fiddly to make but this one is surprisingly straightforward.
You knit it as a flat rectangle, making little short rowed legs, head and tail as you go and then wet felt it down to miniaturise it to fit a walnut shell. Add simple embroidery for the eyes and mouth (I think adding claws would be fun).
You don't need much to make one- a walnut shell, 8m of 4-ply/fingering weight feltable yarn (100% wool, not superwash treated, I used 1.9g of Jamieson's Spindrift in shade 'Leprechaun'), a scrap of embroidery thread, 2.75mm knitting needles, a needle and about 3/4 hour of your life.